Seoul , South Korea -LRB- CNN -RRB- -- South Korean marines detained an American man on the bank of a river bordering North Korea late Tuesday , according to a South Korean Defense Ministry official and a senior U.S. State Department official .

The marines were on a regular patrol mission west of Seoul when they caught the man by the Han River in an area where it divides North and South Korea .

The U.S. citizen was being interrogated Wednesday , the defense ministry official said , adding that it was not immediately known whether the American was trying to cross into North Korea .

The U.S. Embassy in Seoul said it was `` aware of the reports that a U.S. citizen has been detained attempting to swim from South Korea to North Korea . ''

The Embassy said it had been in contact with South Korean authorities but did n't have any additional information to share .

Americans are allowed to travel to North Korea , usually arriving by plane from Beijing . But it 's forbidden to cross from South Korea into North Korea . The U.S. State Department warns citizens against all travel to the authoritarian state led by Kim Jong Un .

Three U.S. citizens are currently in detention in North Korea : Kenneth Bae , Matthew Todd Miller and Jeffrey Fowle .

A North Korean court on Sunday sentenced Miller to six years hard labor for committing `` acts hostile '' to North Korea , although the circumstances surrounding his alleged crime remain murky .

Bae , a Korean-American missionary , is serving a 15-year sentence for allegedly trying to bring down the North Korean government . Fowle , who was arrested in June while traveling as a tourist , is still awaiting trial .

Fast facts on detained Americans

CNN 's K.J. Kwon reported from Seoul , and Jethro Mullen from Hong Kong . CNN 's Paula Hancocks , Elise Labott and Josh Levs contributed to this report .

@highlight

NEW : U.S. Embassy says it 's aware of reports the man was trying to swim to North Korea

@highlight

A South Korean official says not immediately clear if the man was attempting to cross the border

@highlight

He was caught on the bank of the Han River where it divides the two Koreas